The present invention is in a process of carrying out a combined gas turbine and steam turbine process in which the gas turbine process is carried out with a fuel gas which has been produced from solid carbonaceous material and has subsequently been desulfurized, the steam turbine process is carried out with steam which has been produced by the heat generated by the combustion of the carbonaceous gasification residue, and the carbonaceous combustion residue is burnt with oxygen-containing exhaust gases from the gas turbine process.
The energy crises has given rise in recent years to an increasing trend to replace oil and gas by solid fuels, particularly coal, in the generation of electric power. In the production of electric power from solid fuels, greater efforts also have been made to increase the efficiency and the recovery of thermal energy from such fuels in such a manner that the primary energy source is utilized to a higher degree while meeting the more stringent environmental protection requirements. It is known that an increase of the efficiency will result in a decrease of the quantity of pollutant emitted per unit of energy which is produced if given means are used to purify the exhaust gases.
In the production of electric power, improved efficiencies can be achieved by adopting measures in consideration of thermodynamics, particularly in combined gas turbine and steam turbine processes. Whereas a gas turbine may be gas- or oil-fired for that purpose, to achieve a decisive advantage one must supply the gas turbine with a gas which has been produced by a partial gasification of solid fuel.
For instance, in the coal conversion process of VEW, coal is partly gasified in a gasifier, the pollutants are scrubbed from the resulting gas and the scrubbed gas is subsequently burnt in the gas turbine. The coke left after the partial gasification is burnt in the furnace of a steam generator with the oxygen-containing exhaust gases from the gas turbine and the resulting steam is supplied to a steam turbine (K. Weinzierl, "Kohlevergasung zur Wirkungsgradverbesserung im Kraftwerk", VGB-Kraftwerkstechnik 62 (1982), No. 5, pages 365 et seq., and No. 10, pages 852 et seq.).
Whereas the above-mentioned concept of the combined gas turbine and steam turbine process appears to be only attractive at first sight, problems are involved in the technology of the several process steps and in their combination. It must be borne in mind that even drawbacks or disadvantages which affect only details of the process may eliminate the improvement of the efficiency achievable in the process. For instance, a relatively high temperature gasification has the disadvantage that valuable gas produced in the process is spent for air preheating, which is required for the high gasification temperature. Because the gasification temperature and, as a result, also the gas temperature, is high, an appreciable quantity of sensible heat must be extracted from the produced gas. This is usually accomplished by a production of superheated steam, which is supplied to the steam turbine. As a result, the above-mentioned design of the gasification stage involves a shifting of energy from the gas turbine stage to the steam turbine stage and thermodynamic considerations show that a substantial part of the improvement in efficiency is thus consumed.
Another problem is involved in the combustion, for instance, when it is not possible to burn as completely as possible the carbon which is contained in the gasification residue. Great problems, which may also adversely affect the efficiency, are also involved in the desulfurization of the fuel gases produced by the gasification and of the flue gases derived from said fuel gases as well as the flue gases produced by the burning of the residue.
EP-A1-62 363 discloses subjecting carbonaceous material in a first stage to a gasification under a pressure of up to 5 bars and at a temperature from 800.degree. to 1100.degree. C. by a treatment with oxygen-containing gases in the presence of water vapor in a circulating fluidized bed to convert 40 to 80% by weight of the carbon contained in the starting material. Sulfur compounds are removed from the resulting gases at a temperature in the range from 800.degree. to 1000.degree. C. in a suspension. The gas is then cooled and subjected to dust collection, and in a second stage the gasification residue and the by-products obtained by the purification of the gas, such as laden desulfurizing agent, dust, and gas liquor, are supplied to a second circulating fluidized bed, in which the remaining combustible constituents are burnt with an air excess of 1.05 to 1.40 of the stoichiometric demand.
But that proposal has been made with the object to provide power in various forms for the industrial production of certain products, e.g., to provide power in the form of steam for heating purposes or in the form of different fluids at high temperature and in the form of clean fuel gases, which can be burnt without adversely affecting the quality of the product. The degree to which the primary energy (e.g., of coal) is converted to the secondary energy sources consisting of fuel gas and process heat should be variable within wide limits in adaptation to the instantaneous demand for secondary energy in one form or another. This means that the problem solved by the known process outlined hereinbefore does not arise in that form in combined gas turbine and steam turbine processes. This is apparent, inter alia, from the different degrees of gasification.